The present invention relates to analog signal processing and, more particularly, to signal processing in which inphase and quadrature signal components are generated. A major objective of the present invention is to provide an integrated circuit receiver for radio-frequency communications with improved image rejection.
A classical radio communications system includes a transmitter and a receiver. The transmitter modulates the amplitude, frequency, and/or phase of a carrier signal with a signal bearing information (e.g., sound, video, and data) to yield a modulated carrier signal. The receiver receives and demodulates the modulated carrier signal to extract the information-bearing signal.
Typically, a receiver receives not only a desired modulated carrier signal, but also other modulated carrier signals spanning a crowded frequency spectrum. A challenge of such a receiver is to select, i.e., "tune in", a "target" carrier signal while rejecting unwanted signals.
To take advantage of more economical low-frequency components, a received signal is typically converted to a low frequency prior to demodulation. A "heterodyne" receiver typically includes a mixer and a local oscillator. The oscillator outputs a constant-frequency waveform; the oscillator frequency can be adjusted to "tune in" targets with different carrier frequencies. The mixer mixes the incoming signal with the oscillator waveform to yield a difference signal (or, more rarely, a sum signal). The difference signal includes a frequency-converted target signal that bears the same modulation as the as-received target signal; the carrier frequency of the converted target signal is the difference between the carrier frequency of the as-received target signal and the oscillator frequency. Thus, if a received signal having an 805 MHz carrier frequency is mixed with an 800 MHz oscillator waveform, a signal with a 5 MHz carrier frequency results. Thus, the 5 MHz signal can be demodulated instead of the 805 MHz signal.
Typically, unwanted signals that are also received are similarly frequency shifted. Assuming proper bandwidth allocations, these unwanted signals can be distinguished by frequency. Suitable bandpass filters can be used to reject most of the unwanted signals either before or after frequency conversion.
Special consideration must be given to image signals that are converted by mixer stages to carrier frequencies of the same magnitude as the target signal. In the above example, an image signal with an as-received carrier frequency of 795 MHz is converted to a carrier frequency of -5 MHz. Most signal processing devices do not distinguish frequencies by sign, so it can be difficult to remove the -5 MHz image signal from the 5 MHz target signal.
Where the as-received carrier frequencies of the target and image signals are sufficiently different, bandpass filtering can remove the image signal before mixing. Thus, the undesired image signal is not generated at the mixer output. However, it can be impractical to bandpass filter an as-received image signal having a carrier frequency close to that of the target signal. For example, it is typically impractical to bandpass filter out a 795 MHz image signal from an 805 MHz target signal.
Complex filters, also known as "vector" or "polyphase" filters, are known that can reject image signals in favor of target signals. Such a filter has been disclosed in "A Fully Integrated 900 MHz CMOS Double Quadrature Downconverter" by Jan Crols and Michiel Steyaert (ISSCC Digest of Technical Papers and Slide Supplement, Feb. 16, 1995, pp. 100-101, 136-137). Each output of a complex filter is a function of two filter inputs; this function takes into account the phase relationship between the two inputs.
For example, a complex filter can pass a signal in which the inphase component leads the quadrature component while attenuating a signal with the same frequency in which the inphase component lags the quadrature component. If the inputs to such a complex filter are the inphase and quadrature components of a target signal in which the inphase component leads, the filter attenuates the image signal because, in the image signal, the inphase component lags.
While a complex filter can provide sufficient image attenuation in cases where the target signal strength is at least comparable to the image signal strength, there are important applications where this condition does not hold. This is especially the case in mobile receiver applications, where, for example due to an unfavorable difference in transmitter proximity, an image signal can be 60 dB stronger than the target signal. In such cases, image rejection can be achieved using a multistage complex filter, but such a solution is expensive.
More economically, the image signal can be rejected by total cancellation instead of filtering. With the inphase and quadrature components (whether filtered or unfiltered) of the mixer output separated, one of the components can be shifted by one-quarter cycle so that the target signal components add constructively and the image components cancel. If, for example, the inphase component of the target signal leads the quadrature component by one-quarter cycle, the former can be delayed by one-quarter cycle to time-align it with the target quadrature component. When summed, the time-aligned target components add constructively. In the same example, the inphase component of the image signal lags its quadrature component. Delaying the inphase component of the image signal places it in an anti-phase relationship with the quadrature component of the image signal; upon summation, the inphase and quadrature components cancel. Thus, the image signal can be rejected in favor of the target signal.
Generation of the inphase and quadrature components is conventionally performed using two mixers driven in quadrature by the same oscillator. The effectiveness of the image rejection, whether performed by a complex filter, an image rejector, or both, depends on several factors, including the equality of the mixer gains and orthogonality of the inphase and quadrature components. In practice, it is difficult to obtain gain equality between signal mixers. Accordingly, image rejection effectiveness has been limited in systems using conventional mixers.
Parent U.S. patent application Ser. No. 08/321,501, the subject matter of which has been published as Great Britain Patent Application GB 2,294,169, discloses several time-share mixers used in both I-Q demodulators and image-rejecting receivers. In I-Q demodulator embodiments, an input signal, which can be the output of a first mixer stage, is divided into quarter cycle segments corresponding to respective phases of the input signal. The segments include in succession, for example, inphase segments, quadrature segments, inverse-phase segments, and inverse-quadrature segments. (Alternatively, depending on the modulator, the order of the segments can be quadrature, inphase, inverse quadrature, and inverse phase.)
A polarity inverter can be used to invert every other pair of segments so that the inverse-phase segments become inphase segments and the inverse-quadrature segments can become quadrature segments. To this end, the polarity inverter is switched at the frequency of the input signal. The polarity inverter can be inserted at either the signal input, the oscillator input, or the output of the first stage mixer. The result of mixing and polarity inversion is a signal with alternating inphase and quadrature segments.
A distribution switch, operating at twice the rate of the input signal, routes the inphase segments along an "inphase" path and the quadrature segments along a "quadrature" path. This yields the desired separation of inphase and quadrature signal components. The components are in the form of pulses instead of being continuous. However, any filtering used to eliminate unwanted signal frequencies will also eliminate the harmonics associated with the pulse characteristics of the components. In other words, filtering the inphase and quadrature pulse trains yields continuous inphase and quadrature component signals. Thus, the combination of a polarity inverter and a distribution switch can serve as a mixer stage that generates inphase and quadrature components.
In the image-rejecting receiver embodiments, operation is similar. However, in these embodiments, the polarity inverter frequency is offset from the signal frequency by the output frequency of the polarity inverter. Where the polarity inverter output frequency is non zero, the phases of the input signal segments defined by the polarity inverter are not strictly commensurate with quarter-cycle segments of the input signal; there is a disparity due to the frequency offset. However, since in practice the disparity is small and since the distribution switch is switching the segments into "inphase" and "quadrature" signal paths, the terms "inphase", "quadrature", "inverse phase", and "inverse quadrature" are used here and below to distinguish the successive segments, as they are in the I-Q demodulator case.
It can be difficult to ensure that the duration for which the distribution switch distributes the signal along the inphase path exactly matches the duration for which the distribution switch distributes the signal along the quadrature path. If these durations are not matched, the time-averaged gain along the paths will differ. If the time-averaged gains differ, image rejection suffers.
Accordingly, a duty-cycle equalizer switch can be placed upstream of the distribution switch. The duty-cycle equalizer switch alternatively directs and diverts the input signal to and from the distribution switch input. Thus, what reaches the distribution switch is not a continuous signal, but a pulse train of uniformly spaced pulses of equal duration. The switch timings are such that the pulses alternate between "inphase" pulses and "quadrature" pulses. The inphase pulses are segments of the inphase component of the target signal, while the quadrature pulses are segments of the quadrature phase of the target signal.
The distribution switch directs the inphase pulses along an inphase path to define an inphase pulse train; the distribution switch directs the quadrature pulses along a quadrature path to define a quadrature pulse train. The bandpass filtering used to remove unwanted frequencies also serves to smooth the pulses into continuous inphase and quadrature component signals. These can be processed by a complex filter or an image rejector to provide relatively effective image rejection while avoiding the gain matching problems afflicting mixer stage employing nominally matched mixers.
The time-share I-Q mixer with duty-cycle equalization provides for definite improvements in image rejection. However, as this new performance plateau has been achieved, sights are being set on even more effective image rejection.